It is an exciting time in breast cancer immunotherapy, as we are testing a number of novel immunotherapies that can counter a range of immune defects in the cancer immunity cycle in order to supplement the modest benefits obtained from immune checkpoint blockade. Maximizing the impact of cancer immunotherapy on breast cancer outcomes hinges on identifying biomarkers of response and pairing the right treatments with the right patients. While tumor biomarkers have been best validated for clinical use, there is a growing awareness that host variables are important determinants of anti-tumor immunity and can also be potential biomarkers for treatment response. This chapter reviews these host characteristics that are emerging biomarkers of response to breast cancer immunotherapy, with a focus on peripheral blood biomarkers, regional lymph node characteristics, demographic variables (age, sex, body mass index), and gut microbiome. Future strategies to optimize biomarker development are also discussed, including emerging immune profiling technologies, multiparameter biomarkers, timing of tissue collection, and role for collaborative multi-institutional efforts.